The present invention relates to aerial systems and more particularly to aerial systems operating in the induction communication range and comprising at least one receive and transmit aerial pair. The aerial pair may be used in presence sensing or identification systems employing transponders or tags, such as those described in the following patents: G.B. Pat. Nos. 2017454B, 2077556 and 2102250.
A problem with receive/transmit aerial pairs, especially when the aerials of the pair are relatively closely positioned and operating at similar frequencies, is that mutual interference or interaction occurs.
One solution which has been proposed is to screen the receive aerial from the transmitted field. In certain instances, however, screening is not practical or is ineffective.
Another proposed solution to this problem is to arrange the transmitted aerial so that the electric and magnetic components of the transmitted field are orthogonal with respect to the respective components at the receive aerial. However, this solution cannot be applied where the effective axes of the transmit and receive aerials are constrained. This solution may also be inappropriate where a system comprises two aerial pairs. If two pairs are employed with the aerials within each pair being mutually orthogonal but with the pairs having a random orientation with respect to each other, the range of possible resultant combinations of orientations between the two pairs causes complex problems. In a transponder or tag communication system, for example, the deviation from the optimum value of the signal from the control equipment to the tag may permit activation of the tag without being able to detect its response. Such a system might comprise a small hand-held transponder and control equipment with a fixed reading head comprising a receive and transmit aerial pair. The net effect of such a configuration is to reduce the average range performance of the system.
One particular solution is proposed in U.S. Pat. No. 2,779,908, in which the coupling between the transmitting and receiving coils of an inductive train identification system is reduced by a short-circuited neutralizing coil. However, certain limitations are still imposed on the physical relationship between the coils.